Germ cells play a unique role in gamete production, heredity and evolution. Therefore, to understand the mechanisms that specify germ cells is a central challenge in developmental and evolutionary biology. Data from model organisms show that germ cells can be specified either by maternally inherited determinants (preformation) or by inductive signals (epigenesis). Here we review existing data on 28 metazoan phyla, which indicate that although preformation is seen in most model organisms, it is actually the less prevalent mode of germ cell specification, and that epigenetic germ cell specification may be ancestral to the Metazoa.

/pdf/mechanisms-of-germ-cell-specification-across-the-metazoans-30rvm8f91n.pdf

Mechanisms of germ cell specification across the metazoans: epigenesis and preformation

A series of cladistic analyses assesses the status and membership of the taxon Polychaeta. The available literature, and a review by Fauchald & Rouse (1997), on the 80 accepted families of the Polychaeta are used to develop characters and data matrices. As well as the polychaete families, non-polychaete taxa, such as the Echiura, Euarthropoda, Onychophora, Pogonophora (as Frenulata and Vestimentifera), Clitellata, Aeolosomatidae and Potamodrilidae, are included in the analyses. All trees are rooted using the Sipuncula as outgroup. Characters are based on features (where present) such as the prostomium, peristomium, antennae, palps, nuchal organs, parapodia, stomodaeum, segmental organ structure and distribution, circulation and chaetae. A number of analyses are performed, involving different ways of coding and weighting the characters, as well as the number of taxa included. Transformation series are provided for several of these analyses. One of the analyses is chosen to provide a new classification. The Annelida is found to be monophyletic, though weakly supported, and comprises the Clitellata and Polychaeta. The Polychaeta is monophyletic only if taxa such as the Pogonophora, Aeolosomatidae and Potamodrilidae are included and is also weakly supported. The Pogonophora is reduced to the rank of family within the Polychaeta and reverts to the name Siboglinidae Caullery, 1914. The new classification does not use Linnaean categories, and the Polychaeta comprises two clades, the Scolecida and Palpata. The Palpata has the clades Aciculata and Canalipalpata. The Aciculata contains the Phyllodocida and Eunicida. The Canalipalpata has three clades; the Sabellida (including the Siboglinidae) Spionida and Terebellida. The position of a number of families requires further investigation.

Cladistics and polychaetes

The egg the exoskeleton growth and moulting the epidermis musculature the nervous system the pseudocoelom the secretory-excretory system the digestive system the reproductive system nematode pathology.

The Structure of Nematodes

The pericyte--a review.

Insight into the origin and early evolution of the animal phyla requires an understanding of how animal groups are related to one another. Thus, we set out to explore animal phylogeny by analyzing with maximum parsimony 138 morphological characters from 40 metazoan groups, and 304 18S rDNA sequences, both separately and together. Both types of data agree that arthropods are not closely related to annelids: the former group with nematodes and other molting animals (Ecdysozoa), and the latter group with molluscs and other taxa with spiral cleavage. Furthermore, neither brachiopods nor chaetognaths group with deuterostomes; brachiopods are allied with the molluscs and annelids (Lophotrochozoa), whereas chaetognaths are allied with the ecdysozoans. The major discordance between the two types of data concerns the rooting of the bilaterians, and the bilaterian sister-taxon. Morphology suggests that the root is between deuterostomes and protostomes, with ctenophores the bilaterian sister-group, whereas 18S rDNA suggests that the root is within the Lophotrochozoa with acoel flatworms and gnathostomulids as basal bilaterians, and with cnidarians the bilaterian sister-group. We suggest that this basal position of acoels and gnathostomulids is artifactal because for 1,000 replicate phylogenetic analyses with one random sequence as outgroup, the majority root with an acoel flatworm or gnathostomulid as the basal ingroup lineage. When these problematic taxa are eliminated from the matrix, the combined analysis suggests that the root lies between the deuterostomes and protostomes, and Ctenophora is the bilaterian sister-group. We suggest that because chaetognaths and lophophorates, taxa traditionally allied with deuterostomes, occupy basal positions within their respective protostomian clades, deuterostomy most likely represents a suite of characters plesiomorphic for bilaterians.

/pdf/animal-phylogeny-and-the-ancestry-of-bilaterians-inferences-3ir2m11onv.pdf

Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences.

Summary

(1) Based on the classical studies of Goodrich, protonephridia are believed to be phylogenetic antecedents of metanephridia. It is argued here that the primary factor determining the type of nephridium expressed is body size rather than phylogenetic status.



(2) The proposed model defines a nephridium functionally and predicts two general configurations for filtration nephridia in animals.



(3) Application of the model to metanephridial and protonephridial systems indicates differences in the sites of ultrafiltration and mechanisms of pressure generation.



(4) Metanephridial systems function by muscle-mediated filtration of vascular fluid into a coelomic space before modification by an excretory duct.



(5) Protonephridial systems function by cilia-mediated filtration of extracellular fluid into the lumen of a protonephridial terminal cell before modification in an adjoining duct.



(6) The model predicts a correlation between animals with blood vessels and metanephridia, and animals without blood vessels and protonephridia. The correlation is shown to be nearly perfect.



(7) Exceptions to the model are discussed.



(8) Original experimental evidence is given for the permeability of the protonephridial terminal cell to iron dextran and its reabsorption by the protonephridial duct in the polychaete, Glycera dibranchiata.



(9) Experimental data for proto- and metanephridial systems are summarized and shown to support the proposed model.



(10) The ultrastructure of the exceptional amphioxus ‘protonephridium’ is reviewed and original data are presented. Its organization is structurally and perhaps functionally intermediate between proto- and metanephridial systems.



(11) An original ultrastructural comparison is made of monociliated nitration cells in a size range of larval invertebrates from five phyla. Filtration cells that are structurally intermediate between protonephridial solenocytes and metanephridial podocytes are noted in larvae intermediate in body size between the two extremes. The comparative data suggest that (i) podocytes and solenocytes are homologous cells and (ii) that body size is correlated with which of the two designs is expressed.



(12) The fates of larval podocytes are followed through metamorphosis in three species. The results confirm the equivalence of podocytes and solenocytes as suggested by the comparative analysis. They further indicate that which morph is expressed is a function of body design factors discussed in the model.



(13) Protonephridia are believed to be primitive to metanephridia because they occur in presumably primitive animals and in ontogenetic stages of many animals with metanephridia as adults. It is suggested here that the distribution of protonephridia is related to small body size and the lack of blood vessels, regardless of phylogenetic status. The occurrence of protonephridia in the larvae of species with metanephridia as adults is explained similarly as a function of the small larval size and lack of blood vessels.

The functional organization of filtration nephridia

The majority of large, bilaterally-symmetric animals possess two structurally distinct systems of internal fluid transport, the coelomic circulatory system and the blood vascular system (BVS). The coelomic circulatory system is generally situated laterally, lined by a continuous layer of mesodermally-derived cells and moves fluid with cilia or body wall muscles. The BVS occurs in extracellular matrix with the principal vessels situated dorsally and ventrally. The vessels are typically unlined by cells. Fluid is propelled by contraction of overlying muscle cells. Co-occurrence of two systems of internal fluid transport is explained by a segmentation hypothesis: Most large, triploblastic animals are segmented with at least two or three fluid-filled compartments separated by septal bulkheads. Because exchange surfaces (gas, nutrient) are regionally restricted, selective pressure favors a fluid transport system that bridges septal bulkheads. The BVS fulfills this requirement because it is situated in extracellular matrix that is non-cellular and continuous throughout organisms. A survey of animals indicates that all segmented taxa possess a BVS, except Sipuncula, where compensation sacs function as BVS analogues. Loss of the BVS in Hirudinea and other annelids is correlated with the disappearance of septa. Funicular vessels in Bryozoa are compared with BVS in other lophophorates. The fluid transport system of Nemertini and some platyhelminths is shown to conform to the definition of a coelomic circulatory system. This investigation supports Liwanow's proposition that the BVS evolved as the progressive modification of extracellular matrix in triploblastic animals. Evolution of coelomic circulatory systems is unresolved.

Morphology of metazoan circulatory systems

Nemertines are traditionally viewed as acoelomates with blood vascular systems and a specialized cavity housing an eversible proboscis, or, as coelomates with modified coelomic cavities forming the rhynchocoel, lateral vessels and gonadal sacs. Comparative ultrastructural data such as cell polarity, the occurrence of podocytes and mesodermal cells with rudimentary cilia for proboscis, rhynchocoel, lateral and dorsal vessels, and gonadal linings are consistent with their interpretation as modified coelomic cavities as found, analogously, in the higher leeches. These and additional data regarding musculature, connective tissue, glia-like cells and gametes for 13 species affirm that nemertines are intermediate between full-blown coelomate and acoelomate grades of organization. A comparison of ultrastructural details, however, indicates that their acoelomate attributes evolved secondarily from a more typical coelomate condition.

Comparative Ultrastructure and the Evolution of Nemertines

The pore canal-hydropore complex in the larvae of echinoderms and hemichordates has long been recognized as an important character establishing a close phylogenetic relationship between the two phyla. An experimental and ultrastructural analysis of this complex in a tornaria and a bipinnaria larva indicates that it is a functional nephridium. The ciliated pore canal drives a constant, unidirectional efflux of coelomic fluid out of the hydropore. Two percent and 14% ofthe body volume are cleared per hour at the hydropore by a tornaria of Schizocardium brasiliense and a bipinnaria of Asterias forbesi, respectively. Fluid recovery by the coelom is from the blastocoel, the presumptive blood vascular space, across basal lamina and podocytes lining the coelomic cavity suggesting that the discharged fluid is formed by ciliary-driven ultrafiltration. Although invertebrate deuterostomes are believed to lack discrete excretory organs, an analysis of the metamorphosis of the larval nephridia suggests that adult echi...

https://www.jstor.org/stable/pdfplus/1541916.pdf

Nephridia in the larvae of hemichordates and echinoderms

The heart-kidney of Saccoglossus kowalevskii, which is situated within the anterior preoral proboscis coelom (protocoel), consists of the stomochord, pericardium, heart sinus, and glomerulus. The stomochord, a diverticulum of the gut, is characterized by vacuolated epithelial cells surrounded by basal lamina and connective tissue. The pericardium, a myoepithelium, lies dorsal to the central heart sinus. Opening into the protocoel and connecting with the outside via the proboscis pore is the protocoel duct, which is, in part, composed of multiciliated absorptive epithelial cells. Perfusion of the dorsal trunk vessel with vital dyes reveals a rapid flow of blood into the glomerular blood vessels. Examination of the permeability characteristics of the extracellular matrix underlying the glomerular podocytes reveals the movement of iron dextran (mol. wt 5000 daltons) from the central heart sinus into the protocoel. Iron dextran uptake by glomerular cells and protocoel lining cells is demonstrated. These results suggest that vascular fluid is filtered by the glomerulus, producing a primary urine in the protocoel which may be modified as it passes over the peritoneum, through the protocoel duct, and out of the proboscis pore. New data concerning the morphology of the stomochord are presented. The controversial homology between the hemichordate stomochord and the chordate notochord is addressed.

Structure, Ultrastructure, and Function of the Preoral Heart‐Kidney in Saccoglossus kowalevskii (Hemichordata, Enteropneusta) Including New Data on the Stomochord

Edward E. Ruppert

Papers

The functional organization of filtration nephridia

Morphology of metazoan circulatory systems

Comparative Ultrastructure and the Evolution of Nemertines

Nephridia in the larvae of hemichordates and echinoderms

Structure, Ultrastructure, and Function of the Preoral Heart‐Kidney in Saccoglossus kowalevskii (Hemichordata, Enteropneusta) Including New Data on the Stomochord